Here's what my Autocoms do.... There is a red button on the left handlebar with a small microswitch underneath.
With the microswitch in the middle, it's 'standard PTT operation' , namely you press the red button and the radio transmits whatever you say over the channel for as long as you hold the button pressed. No-one else can transmit while you hold that button down.
With the microswitch set to the right, it's 'Vox' operation, namely you have to say something to cause the intercom to open the channel for transmission and it will stay open as long as you're saying something at a high enough volume to keep the 'Vox' in effect.
With the microswitch set to the left, you open the channel for 'constant transmit', which is quite useful for CBTs (where you talk an awful lot in the beginning) and advanced demo 'commentary' rides, as you just set it and forget it.
I know yours is a Starcom, but 'PTT', 'Vox' and 'Constant talk' will almost certainly work in the same way if your unit has those facilities. The switches themselves may well be different.
Hope that helps. Norman